The accurate centering of a flexible disc with respect to a driving spindle is required for satisfactory operation of a disc drive mechanism. One known method for centering the disc includes use of a rotating spindle capable of supporting the disc by gravity, and an upwardly diverging clamping cone insertable into an opening provided in the top surface of the spindle. A flexible disc, enclosed in a jacket, is positioned between the cone and spindle in accordance with the outside dimensions of the jacket. Due to tolerances between the jacket and the platform adapted to receive it, the jacket and disc are often not axially aligned with the spindle. A second and more serious cause of misalignment is flotation, or slippage of the disc within its jacket.
To center the disc, an opening is provided in the spindle extended downwardly from the top and of equal size to a drive hole provided in the disc. A clamping cone, mounted above the spindle, has a bottom diameter substantially smaller than that of the drive hole and diverges upward to a diameter substantially equal to that of the drive hole. As the cone is inserted downwardly through the drive hole and into the spindle opening, a portion of its inclined surface engages the innermost portion of the off-center disc. Continued descent of the cone causes the disc to slide, moving the innermost rim of the disk radially outward from the spindle center. When the cone is fully inserted into the spindle opening, the drive hole is aligned with the spindle opening so that the disc is centered.
One problem of such a centering method is that the descending cone can permanently bend the inner rim of the disc adjacent the drive hole. Bending occurs whenever friction between the disc and jacket inhibits sliding of the disc responsive to pressure from the cone. The minutely thin disc is extremely susceptible to such bending. Once permanently bent, the disc cannot be accurately centered. A further problem occurs if the spindle and disc are rotating as the non-rotating cone descends. As the cone first engages the disc and spindle, slippage between the disc and spindle and between the disc and cone causes scuffing and other damage to the disc rim. This can enlarge the drive hole and cause off-center revolution of the disc, making data difficult to read. A sufficiently off-center disc could rub against a portion of the jacket, wrinkling or tearing the same and possibly jamming the drive apparatus and causing loss of data.